In order for a compound to function as a plasticizer for polyvinyl chloride (PVC) it must have sufficient affinity for the PVC to resist migration from the internal regions to the surface, however, some degree of mobility must be maintained within the polymer structure if flexibility is to be achieved. A proper balance between these two conflicting considerations must be maintained for effective and useful plasticization of PVC. The problem of the selection of a suitable plasticizer for PVC is further complicated when the plasticized PVC will come in contact with another dissimilar polymeric material. In these situations the plasticizer should not have affinity for the second polymer, otherwise the plasticizer will migrate to the surface of the PVC and into the other polymer substrate. In addition to reducing the amount of plasticizer in the PVC and thus detracting from the physical properties this uncontrolled migration can be even more detrimental to the other polymeric material. Migration of plasticizer to certain elastomeric polymers can for example, cause swelling of the elastomer. In the case of plasticizer migration to rigid materials stress cracking and embrittlement commonly occur.
Plasticizer migration is particularly troublesome in situations where plasticized PVC is contacted with polystyrene and acrylonitrile-butadienestyrene (ABS) resins. With ABS, marring of the ABS surface at the interface is common when conventional easy-processing type PVC plasticizers are used. The surface of the ABS becomes dull and is often accompanied by noticeable softening and swelling. In addition to affecting the appearance adversely, the polymer may be affected internally as evidenced by deterioration of the physical properties. Stress cracking (the formation of numerous tiny internal and surface cracks aligned in the same direction and usually arranged in more or less parallel plains) is the result of plasticizer migration and is attended by a loss in the physical strength of the polymer. Polystyrene resins are particularly prone to stress cracking (crazing) if they are placed in contact with PVC compositions plasticized with known easy-processing plasticizer types.
In view of the above-mentioned difficulties it would be highly desirable and advantageous to develop plasticizers for PVC which are readily processable and compatible with PVC but are essentially non-migrating. It would be even more desirable if these plasticizers had little or no affinity for ABS or polystyrene resins.